Various types of catalytic hydrocarbon conversion reaction systems have found widespread utilization throughout the petroleum and petrochemical industries for effecting the conversion of hydrocarbons to different products. Moreover, such systems often result in either the net production or the net consumption of hydrogen. As applied to petroleum refining, these reaction systems have been employed to effect numerous hydrocarbon conversion reactions, including catalytic reforming and catalytic dehydrogenation of paraffins.
Catalytic dehydrogenation of C.sub.2 -C.sub.5 hydrocarbons is well known in the petroleum industry. The monoolefinic hydrocarbon products derived therefrom are generally useful as intermediates in the production of other more valuable hydrocarbon conversion products.
Catalytic dehydrogenation can be combined with other catalytic hydrocarbon conversion processes to produce a variety of useful products. For example, the olefins produced during catalytic dehydrogenation of a liquid petroleum gas stream containing isobutane can be used in conjunction with an etherification unit wherein isobutylene is reacted with methanol to produce methyl-t-butyl ether (MTBE). Another example of combining catalytic dehydrogenation of hydrocarbons with other hydrocarbon conversion processes is the use of propylene and butylenes produced from dehydrogenation in an HF alkylation unit wherein these olefins are alkylated with isobutane to produce a high octane motor fuel.
The separation of a hydrogen-rich gas stream from the effluent of a catalytic hydrocarbon conversion process is well known in the art. It is important to separate the hydrogen-rich gas stream from the catalytic conversion effluent for several reasons:
(1) catalytic conversion reactions generally require the presence of hydrogen and recycling the processed-derived, hydrogen-rich gas stream to the catalytic conversion reaction zone is cost effective;
(2) any excess processed-derived, hydrogen-rich gas can be used in other catalytic hydrocarbon conversion processes located at the refinery; and
(3) it is particularly desirable not to lose product olefins or unreacted feed hydrocarbons in the product hydrogen.
An example of a process for separating a hydrogen-rich gas stream from a catalytic reforming effluent can be found in U.S. Pat. No. 3,520,799 (Forbes). This patent discloses a method of obtaining a high purity hydrogen gas stream from a catalytic reforming effluent by passing the effluent to a low pressure vapor-liquid equilibrium separation zone from which there is produced a hydrogen-containing gas stream and a liquid hydrocarbon stream. After compression, the hydrocarbon-containing gas stream is recontacted with the liquid hydrocarbon stream and the resulting mixture is passed to a high pressure vapor-liquid equilibrium separation zone. A second hydrogen-containing gas stream is produced having a higher hydrogen purity than the first. A portion of this second hydrogen-containing gas stream is passed into an absorption zone where it is contacted with a lean sponge oil, preferably comprising C.sub.6.sup.+ hydrocarbons. A third hydrogen-containing gas stream is removed from the absorption zone and, after cooling, passed to a third vapor-liquid equilibrium separation zone. The sponge oil is removed from the absorption zone and is admixed with the liquid hydrocarbon stream from the low pressure vapor-liquid equilibrium separation zone prior to recontacting thereof with the compressed hydrogen-containing gas stream. A hydrogen-rich gas stream is removed from the third vapor-liquid equilibrium separation zone.
U.S. Pat. No. 3,882,014 (Monday et al.) also discloses a method of obtaining a high purity hydrogen gas stream from a catalytic reforming effluent. The effluent is first passed to a vapor-liquid equilibrium separation zone from which there is recovered a liquid hydrocarbon stream and a hydrogen-containing gas stream. After compression, the hydrogen-containing gas stream is passed to an absorption zone wherein it is contacted with a sponge oil comprising stabilized reformate. A hydrogen-rich gas stream is recovered from the absorption zone with one portion thereof being recycled to the reforming zone while the remainder is recovered for use in other hydrocarbon conversion processes.
U.S. Pat. No. 4,212,726 (Mayes) discloses another method of recovering hydrogen-rich gas streams from catalytic reforming reaction zone effluents wherein the reaction zone effluents from the catalytic reforming process are passed to a first vapor-liquid equilibrium separation zone from which is recovered a first hydrocarbon liquid stream and a first hydrogen-containing gas stream. After compression, the hydrogen-containing gas stream is passed to an absorption column where it is contacted with the first liquid hydrocarbon from the vapor-liquid equilibrium separation zone and stabilized reformate. A hydrogen-rich gas stream is recovered from the absorption zone with one portion being recycled back to the catalytic reforming reaction zone and the balance being recovered for use in other hydrocarbon conversion processes.
In all of the above patented processes, the catalytic hydrocarbon conversion effluent from which the hydrogen-rich gas stream is recovered is an effluent from a catalytic reforming reaction zone whereas in the present invention the catalytic hydrocarbon conversion effluent from which the hydrogen-rich gas stream is recovered is an effluent from a catalytic dehydrogenation reaction zone. There are significant differences in reactions, feedstocks, operating conditions and effluents between reforming and dehydrogenation processes.
Catalytic reforming reactions are numerous and varied. For example, the catalyst and operating conditions used in reforming promote the formation of higher octane unsaturated cyclic compounds such as aromatics by dehydrogenation of naphthenes, isomerization of paraffins and naphthenes, dehydrocyclization of paraffins, and hydrocracking. However, in a catalytic dehydrogenation zone, only one reaction is predominant, that reaction being dehydrogenation of paraffins to produce olefins.
Reforming feedstocks contain a mixture of hydrocarbon components that typically have a boiling point range of about 100.degree. F. to about 400.degree. F. In contrast, dehydrogenation feedstocks are typically made up of pure components of methane (b.p. -127.5.degree. F.), propane (b.p. -43.7.degree. F.), isobutane (b.p. 10.9.degree. F.) and isopentane (b.p. 82.1.degree. F.), each having much lower boiling points.
The effluent from a reforming reaction zone contains a significant amount of normally liquid hydrocarbons such as benzene, toluene and xylenes. Accordingly, a suitable separation of the hydrogen-rich gas stream from the catalytic hydrocarbon conversion effluent can generally be effected by condensing out the hydrocarbons and absorbing the hydrogen-containing gas with lean oil at relatively mild conditions of temperature and pressure. For instance, in the Forbes and Mayes patents, the absorber temperatures are about 90.degree.-150.degree. F. Further, in the Monday et al. patent, the absorber temperature is about 100.degree. F.
In contrast, the dehydrogenation effluent contains a significant amount of lower molecular weight olefinic hydrocarbons that are normally in the gaseous state. Accordingly, the operating conditions, particularly the absorber temperature, must be substantially lower to accomplish effective separation of a hydrogen-rich gas stream from a dehydrogenation effluent.
U.S. Pat. No. 4,381,418 (Gewartowski et al.) discloses a process for recovering a hydrogen-rich gas stream from the effluent of a catalytic dehydrogenation reaction zone comprising compressing the dehydrogenation effluent stream and cooling by indirect heat exchange using catalytic dehydrogenation feedstock comprising a hydrogen/hydrocarbon admixture, forming a hydrogen-containing gas stream and a liquid hydrocarbon stream, separating the hydrogen-containing gas stream and the liquid hydrocarbon stream, cooling the hydrogen-rich gas stream by gas expansion to form a hydrogen-rich gas stream, combining one portion of the hydrogen-rich gas stream with a paraffinic hydrocarbon stream to form the catalytic dehydrogenation feedstock admixture referred to above and recovering the other portion of said hydrogen-rich gas stream. Nowhere in Gewartowski et al. is there disclosed or suggested contacting a hydrogen-containing gas stream with a liquid absorbent.